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Would it be possible to put wind turbines alongside runways that are driven by the exhaust of planes?
The only thing somewhat similar that I could find is research from students to use air vents from factories. This is of course a much lower force to deal with.
I'm curious if someone could give some advice or could tell me why it would or wouldn't work.

I've actually heard it proposed to do something similar along highways.

@Fred Larson:  From that article "Natural wind speed normally does not exceed several miles an hour."  They sure don't live around here, where it's quite commonplace for winds to be strong enough to flip semis: https://carsonnow.org/story/06/09/2021/wind-flips-semi-washoe-valley-traffic-stopped-northbound-i-580-south-bowers-exit  I've often thought a line of wind turbines along that highway would be quite useful.

The list of things that are possible, yet are not good ideas, is virtually infinite.

Yeah, make them really high, too!

Electricity generated per day: $0.02 worth. Cost of recovering from a fully loaded 747 hitting a wind turbine: $xx,000,000

Hey, we could find a way to harness the kinetic energy of everyone typing on keyboards, just think of how energy is produced just through typing. I think I read it only takes 100 novels to heat a cup of tea. There are a lot of these ideas that sound good, but don't work out in reality. Placing wind turbines in optimal areas would produce far more electricity than an occasional increase in wind from several planes taking off every hour.

@Issel: OTOH, if you have a CPU with really low power consumption, you wouldn't have to worry about your laptop battery dying on you :-)

No, there's no practical benefit to putting wind turbines alongside runways and plenty of drawbacks. Airplane engines and propellers direct their thrust straight back, so the only logical place to put a turbine for energy reclamation would be directly behind the runway threshold to catch the wind from engines as they are spooled up for takeoff.
In order to successfully catch energy behind a departing airplane wind turbines would have to be very close to the engines. There are several problems with that:

Safety: remember that almost all airstrips are used both directions depending on wind, and many runways have an overshoot area past the threshold on the other end which is used in emergencies. Putting wind turbines at the thresholds places an obstacle between a landing airplane and the overshoot area which would be a serious safety risk. The only way this would work is if the turbines are retractable
Turbine Design: wind turbine blades have to be pitched correctly in order to efficiently produce power, the profile of the wind they would capture is a brief, powerful burst which then tails off. A simple turbine with fixed blades would need to be optimized for that brief burst, if you want to catch more of the energy as the airplane rolls away you'd have to have a much more complex variable pitch system. Complexity adds cost
Economics: a wind turbine right behind the threshold would get a few seconds of wind every time. Let's be generous and say each takeoff will produce 10 seconds of wind for power generation, and it's a relatively busy airport with 500 takeoffs per day. That's 5000 seconds of power per day, or about 83 minutes per day. It's doubtful you would be able to make any wind turbine profitable on 83 minutes of wind per day, especially since you'd need these to be retractable and you'd need 2 sets - one for each end of the strip

As for putting turbines to the sides of the runway you would catch very limited amounts of energy, at the same time you'd be installing obstacles and increasing risks. Turbines also cause turbulence which would make crosswind takeoffs more risky.

Two other constraints: All obstacles along the runway must be frangible. and interferences created by blades on VOR and ILS.

To add a few other numbers, say an average twin-engined jet has 60MW of total engine capacity and let's say you could capture even 10% of that, which is absurdly generous, for those 83 minutes.  At a typical market rate of $17 per MWh you would be making about $141 per day.  More likely you'd be lucky to capture 1% of that energy for a whopping revenue of about fifteen bucks a day.

@J 0.1%? 0.01%?

@Peter-ReinstateMonica Yeah, floor's the limit.  I said lucky, after all ;)

... so, mount the turbine onto the tail of the plane, where it can continuously recapture the output of the engines and use it to recharge batteries to extend range... (only kidding! :) )

My god @JeremyFriesner, I think you've got it!

@JeremyFriesner hmm... I smell a RAT:)

Safety would be a big issue. The runways are where planes take off - and land. Large windmills next to the runway would not be a good thimg.
There is quite some energy expelled from the engines, and catching all of it on a busy airport would be worth looking at were it not for:

The main jetstreams are flowing along the runway itself, where there may not be any windmills!
The exhaust flow is close to the ground, which decelerates it.

A better set-up for reducing TO and landing energy would be to imitate aircraft carrier launch & arrest, in a gentler, more comfortable way of course. Airbus made a case for this some time ago, as mentioned in this answer.

Daft as it may seem... https://www.airteamimages.com/pics/250/250521_800.jpg

Energy is expelled on the ground for only a brief period, vs. in-flight.

Maybe it would be better for efficiency to install reflectors to redirect the aircraft's exhaust back at the aircraft, giving it an extra push.

I had many thoughts in the past on how to recover part of the energy dissipated by a landing aircraft. The best idea I came up with is to install on runways an arresting gear (like the ones used in aircraft carriers) connected to a power generating turbine.
Of course there should be some sort of construction standards that all (heavy) aircraft should implement, but the idea is to let the arresting gear to brake the landing aircraft until it comes to a stop; this device could be used in place of air-brakes, gear-brakes, and thrust reversals.

Yeah arresting gear would help,at what cost though? A lot of financial investment, safety may not be impacted, and we’re catching the plane at its lowest airborne energy state.

@Koyovis it doesn't look a massive investment to me; true, it's the lowest energy state, but still a 737-800 (170 tons landing at 140kts) has 500MJ of kinetic energy that can spin an alternator long enough until the next landing in a busy airport. With a landing every 5 minutes you can produce (ideally) ~150MW, like a mid-size hydroelectric power plant. To me it's definitely worth a serious study

@Jack This has been discussed here: Why don't airports use arresting gears to recover energy from landing passenger planes? Also, a 737-800 for sure does not land at 170 t (MTOW is only 79 t, MLW probably lower).

Thanks @Bianfable

Turbochargers are the practical implementation of this idea.

A turbocharger uses the exhaust gases to spin the turbine which then forces more air into your car's engine and increases the engine's power.'
According to the law of thermodynamics; 'energy can neither be created nor destroyed, only altered in form

In the case of planes, already, most of the energy from the fuel is converted into useful mechanical energy used to fly it. The idea is to get the maximum energy in the desirable form, i.e. electrical or mechanical energy. So, essentially, one might think of the turbocharger as the wind turbine installed on the vehicle itself.
Specifically in cars with turbochargers, it uses the exhaust as well as the car's air intake as well to increase its power.

That's not the same thing! If you extract more kinetic energy from a jet engine exhaust, you reduce the thrust. But the airplane needs the thrust for takeoff. Placing wind turbines on the ground would recover some of the energy without reducing thrust.

@Bianfable - It's actually rather comparable.  Both solutions improve thrust in their own way.  Turbochargers make the engine smaller/lighter at the same maximum thrust, thus improving fuel economy.  The turbine on the ground creates a horizontal ground effect, thus improving thrust.  Unfortunately, only for the short while its blade stands directly behind the engine.  That's why turbochargers are more popular than the wind turbines.

Turnochargers are used in piston engines, not in turbine engines like a turbofan.

A turbofan is, for all intents and purposes, already turbocharged.  That is what the "turbo" means.  The exhaust drives the turbine, which via a driveshaft drives the compressor.

Other people have explained a lot of the issues with this system, but assuming you could finally build a system that works with all these restrictions in place and manages to produce power...
You would produce orders of magnitude more power, cheaper and more efficiently, simply by directly burning the kerosene that these planes use to take off and fly and using the generated energy to power a generator. It would probably also be better for the environment.

That may be true, but the kerosene is being burned already. This question is asking about whether that energy (which is already being used) can be reclaimed.

@randomhead I'm sorry, but that wasn't obvious for me. At best it was implied.

“by directly burning the kerosene that these planes use to take off and fly and using the generated energy to power a generator”: But then the planes wouldn't fly!  :)


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Identification of control column spade
I have a control column head similar to that used on the Spitfire and Hurricane but with two levers for cables coming up from the stick. I would like to find out what aircraft used this type.
Markings on the side  are 444/35,  AK-2 and AK-8 in an oval and A1D 4 in a partially stamped circle.

Could you post a photo?

D. H. Mosquito?

Welcome to aviation.SE! A photo would be very helpful here.

The bomber Mosquito had a W wheel, and the fighter version had kind of an L shaped extension of the column, similar to a Spit's spade control, but with a pistol grip.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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Are Belarusian ATC recordings available for Ryanair Flight 4978?
I am interested in locating the ATC recordings of Ryanair Flight FR4978, LGAV to EYVI, 23/5/2021, that had a forced diversion to UMMS in order to arrest a passenger.
I have looked online but couldn't find any recordings available, the closest I found was https://www.liveatc.net/, but wasn't able to find the relevant files, because I don't know the ARTCCs of the controllers they were talking to.

I think you and about a couple of hundred or so media organizations are probably trying to get a transcript of this right now!

Belarussian Department of Aviation released a transcript, see the bottom of http://caa.gov.by/ru/news-ru/view/1-203/ (website is overloaded, here is a copy: https://pastebin.com/ZZXrES84). Not a recording and not an impartial source, so I'm making it a comment instead of an answer.

@aland Neither of those work for me, unfortunately. It may be my location, however.

Why is the question closed? The message says it's "not about aviation", but it clearly is. Would a rollback to revision 1 help, to make it more general?

@ymb1: That's why I mentioned revision 1, there I asked how can one locate the ATC recording, not "please locate this for me". Feel free to add an answer to the meta question about this though.

The full transcript is now available here in Russian: https://mediazona.by/chronicle/FR4978

Not every country allows recording and publishing of ATC communication. The UK for example does not: Is it illegal for Joe Public to listen to ATC in the UK?
While I don't know for sure, I'm guessing Belarus does not allow it. LiveATC has a coverage map, which shows the destination airport EYVI (Vilnius) is covered, but nothing in Belarus is:

I therefore doubt you will be able to get the recording. Since the incident is currently being investigated by multiple countries and organizations, a recording or a transcript might be available in the future as part of the investigation report.

Ok, thanks, so the only option is to wait for whatever media is published by the authorities. I'm gussing one shouldn't hold his breath for this though, since the authorities responsible for the incident are in Belarus.

@user000001 the records will be available in the planes blackbox. It shouldn't take too long for them to be published.

@JonathanReez a standard CVR only keeps the last 2 hours. Don't know about the one in that specific aircraft, but if it's indeed 2 hours there won't be anything interesting on it. Also I'm not sure CVR data can be extracted outside of accident investigations.

an excerpt, maybe carefully chosen, released by a Russian TV channel, dubbed in Russian though...

I cannot play videos at my location, so al I see is the sentence you wrote. Could you transcribe?

This is essentially a link-only answer. The help center advises: "Always quote the most relevant part of an important link, in case the external resource is unreachable or goes permanently offline."

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Why would it not be OK to replace a map light bulb with an LED?
In the question about if it's OK to change your own map light, why did JScarry say "don't you dare replace [your map light] with an LED"? Or was he kidding?

When I made the comment a lot of people were replacing their GE 4509 lights with LEDs. The approved (STCd or PMAd) versions were $ 235 & the versions sold at Tractor Supply were $ 12. People were claiming that you didn't need an STC since they were similar & cheap ones were legal. I disagree. I had an LED in the house overheat & start smoking because the heat generated by the base was trapped in a rubber gasket. So there is a risk that a random LED won’t be safe in your airplane. They sell aircraft versions of map light LEDs now, so even though they are more expensive, that’s what I would use.

White LED spectra contains quite a bit of blue range wavelength, which is bad for night vision.  Red light is much better.
The eyes have cone receptors for day time and rod receptors for night vision.  The rods contain rhodopsin, which enables night vision.  It takes about 1/2 hour for enough to build up to have full night vision.
Exposure to shorter wavelengths quickly breaks down rhodopsin, which is why blue light is to be avoided.
Natural sunlight is strong in blue (and UV) spectra.  Incandescent tungsten filaments have much less blue/green spectra, and a lot more orange/red.  So it may be a bit "old school" to shun LEDs.
Newer OLEDs have spectra closer to candles and incandescent.
One interesting measure is how much the light suppresses melatonin, which helps one sleep.  Newer OLEDs were found to be only fractional of blue light, so it would be important to be sure any replacement map light is appropriate for the task.

Red LEDs are available.

Red LEDs are monochromatic. You can read a text fine under one, but not a color-coded map (so it's ok for IFR plates, but not VFR sectionals).

Many white LEDs have a high colour temperature, which means they have relatively more blue light than typical filament lamps.  6500K is equivalent to daylight, 4000K or higher is often used because it’s more efficient than lower colour temperatures, and gives better colour rendition if you’re not worried about night vision.  It is possible to get lower colour temperatures, but they aren’t easy to find in smaller lights.  2400K would be good if you can find any.  Meanwhile, potential issues with blue light are being addressed; this is an active area of research in the lighting industry.

@Frog some of the older white LEDs were more like blue & yellow, with particularly poor colour rendering in the red/green as well as a strong blue spike. The phosphors used to convert the underlying deep blue emission to white have improved a lot.  There's nothing insurmountable; there are now "warm white" car interior bulbs as well as single emitters, but availability of something of the right fitting and voltage may be problematic

A non-aviation anecdote that may be relevant to reading color-coded charts etc: I have some cuttings of plants that are on my kitchen window sill. In daylight, they look their normal healthy dark blue-green color. In artificial light from the ceiling LED, the new growth does not look green at all, but bright yellow.

I've actually tested this with sectionals. They become quite unreadable with pretty much any single color LED's. Combining two or even three colors renders the chart less readable than incandescent light. LED's typically have extremely narrow (and spiked) spectrum bandwidths.

@Jpe61 that's true for almost all but white (from a single emitter); even RGB white can be a problem as you've found.  The phosphor in a decent white LED has a broadband emission like fluorescent lamps have, but ensuring you have a decent LED means looking up the datasheet for the actual emitter and that's not done for general-purpose bulb.  A bulb used for aviation would of course be expected to have the relevant data available, though this could just be the colour rendering index and not the spectrum

Why the quotes around "organic"? It doesn't mean anything like "free range" or "no pesticides", it literally means that the active layers are made of organic molecules instead of an inorganic semiconductor crystal. Also light bulbs based on OLEDs aren't really common.

Most people don't know @TooTea's mentions of organic. But not all LEDs are the same. I've had good luck with an amber LED (battery-operated finger light) to read sectionals at night. LEDs come in many colors. There are even white LED flashlights that can adjust the color temperature. But I suspect it would be difficult to find something like that in an avionics bulb.

@Suncat2000 "amber" in LEDs can mean a few things - a narrow orangey-yellow that will make all other colours look black, an unusual phosphor-coated broadband emitter mimicking an incandescent bulb run cool (good), or as I have here, a fairly-narrow-band phosphor used for pretty lighting that just happens to suit my science.  In other words YMMV

@TooTea +1. This is the only proper definition of organic, in fact. All the other ways you mention are always pure quackery, incompentence or plain old hogwashing.

I'm surprised there aren't any aviation LEDs available. It's not particularly hard to make an RGB LED that has a much more natural spectrum than pure red/white has.

Quite simply because a random LED bulb is not certified for operation in an aircraft, and the light holder is not certified to operate with a LED bulb.
In practice it would probably work fine, but it is not out of the question that a LED bulb (sometimes containing internal SMPS power supply) might emit electrical noise, or perhaps even fail in ways that a normal light bulb wouldn't.

Owners can manufacture parts for their type certificated airplanes, so the fact that a specific component is not certified does not preclude it from being used.

@Kenn It doesn't seem like "manufacturing parts for" would include a change of technology.

This is the answer, it's not certified. All fun and games until the EMI from the cheapo LED driver interferes with your instruments!

@Harper-ReinstateMonica, LEDs themselves do not make EMI noise, which I think you recognize when you refer to the LED driver. Not all LEDs require drivers, and not all LED drivers make noise. In the case of a map light, where brightness and efficiency are not primordial and the bulb circuit is properly fused, then a driverless 12V led would be a fine substitute. The FAA is very clear on these kind of minor repairs/alterations, and on an owner's authority to sign off on them. And this is before we even question whether the plane is Experimental or Type Certificated.

@KennSebesta While your technology grasp is solid, it depends on something the vast majority of aviators simply do not have: a clear understanding of the difference between a bare LED semiconductor emitter, and an "LED Product" consisting of LED emitters and some sort of electronics.

Agreed, however it remains that @jpa's answer is wrong in a global sense, as it employs the logical fallacy of "not everyone can understand the difference therefore no one can".

Well I'm still not entirely bought into the idea that the "manufacture parts" allowance applies to busting into new technology zones.  Can you replace an approved nickel cadmium starting battery with a homebrew lithium job built out of 18650s and nickel strips?

@Harper-ReinstateMonica that's a fair question, although I don't think that in the case of building a battery you could claim that it's a minor alteration. There's a lot of power in a battery and it can cause a nasty fire, whereas a properly fused map light cannot even in the event of total failure. See https://aviation.stackexchange.com/a/87355/20394 for the current state of affairs for owner maintenance.

I have incandescent DRL (daytime running lights) on my car, which I'd like to replace with (white) LEDs. I've been warned that this will upset something in the electrical system due to the lower power draw, causing the "light bulb is out" warning light to come on. Perhaps the concern about the map light is something similar?

My interpretation of the comment was that replacing the bulb with an LED is going to draw attention to the fact that you have broken the rules. If you are going to go rogue on a minor thing like this, give the inspector some plausible deniability.
Sort of like if you buy some custom aftermarket taillights for your car that maybe aren't officially DOT approved. If you buy red ones the cop probably won't notice and wouldn't care much anyway (unless you're a jerk or have a dozen other equipment violations). If you buy green ones, the cop won't be able to overlook it or let you get away with it.
The other answers here about night vision etc. I think are good advice also.

Unless a specific type of part is governed by the CFRs, this is a gray area of the law. Your example of a green auto taillight is a violation of the law (that expressly says "red" lights on the rear of the vehicle), but not everything is so clear-cut or a "fact".

@Suncat2000 you may very well be correct, but Pondlife has the top rated answer on the linked question and they disagree with you. That being said, I really don't have the expertise to take sides between the two of you. As far as my answer goes, what i was trying to get at with my red and green examples was "superficially appears to be legal even if not" vs. "obviously not legal without even looking closely."

Physical reasons for not changing to LED include

Lower heat output - which sounds good unless there's something else dependent on the warmth.  LED traffic signals in cold locations require heaters to melt ice, whereas the old 420V incandescent lamps keep themselves warm and ice-free.
Lower power draw means less current - again plausibly good, unless there's something measuring the current.  Example - indicators (blinkers) in a car may not cycle between on/off if they are LEDs  The unit that does the blinking in older cars is thermal, and depends on the current flow to switch.
Insurance - In the event of a claim, an insurer will take every possible option to reject the claim.  If you've got anything out of specification the insurance adjuster will look for it, and potentially not pay out.  Doesn't matter if its your insurer or the other person's insurer.  And they know what to look for, its their job to find ways to not pay on claims.


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61.133(b)(1) what counts as a full cross country flight?
14 CFR 61.133(b)(1) states that a commercial pilot without an instrument rating will receive a limitation of “The carriage of passengers for hire in (airplanes) (powered-lifts) on cross-country flights in excess of 50 nautical miles or at night is prohibited."
In a scenario where you are flying your passengers, could you go to an airport 40nm from the first take off airport refuel and then go from there to another 40nm away, for a grand total of 80nm from original take off airport?

@757toga You're not wrong, but the FAA is known for taking a dim view of attempts to circumvent, cheat,  exploit loopholes in, or otherwise go against the spirit of regulations. And courts are known for taking very broad interpretations of those regulations. In this example, a court could rule that this is one flight and that its intermediate stops are irrelevant. If you're going to try this, you'd better have a good lawyer and a favorable FSDO interpretation letter for him to wave around.

Related: https://aviation.stackexchange.com/questions/55825/what-defines-the-end-of-a-cross-country-flight-for-ppl-aeronautical-experience

There is no definition for the word "flight" in the FARs. I'm not aware of any FAA document that clarifies their intent.
The closest I was able to find was only tangential: Clarification of Flight, Duty, and Rest Requirements. In that document, there is discussed "flights" and "flight segments". As an example, there is this passage:

SWAPA provided an example of a 4-leg FDP with a 9-hour flight-time limit in which the crew realizes, after Leg 2, that their total flight time will be 9 hours and 5 minutes if they complete the remaining two legs. SWAPA then asked whether the fligthcrew can depart on Leg 3 of this FDP. In response, the FAA notes that if completing Leg 3 of the scheduled FDP will not cause the flightcrew to exceed the 9-hour flight-time limit, then the flightcrew can take off on Leg 3.

This is typical, where "flight" is mainly used as an adjective, and "flight segment" or "flight leg" (or even just "leg") is used as the noun to describe the actual activity. This example seems to suggest that each leg is a separate flight.
There is also, of course, the definition of "flight time", mainly used to govern logging:

Flight time means:
(1) Pilot time that commences when an aircraft moves under its own power for the purpose of flight and ends when the aircraft comes to rest after landing; or
(2) For a glider without self-launch capability, pilot time that commences when the glider is towed for the purpose of flight and ends when the glider comes to rest after landing.

It could be argued logically that if the time of flight is only that time between starting to move and finally coming to rest after landing, that the actual flight is itself just the movement between those two moments in time.
That said, the FAA is granted broad latitude by the NTSB (the primary adjudication body for contested enforcement actions) in their interpretation of their own rules, and it takes a very dim view of actions taken by pilots that can be perceived as intended solely for the purpose of circumventing those rules.
It is my own opinion, worth every bit of money you paid for it, that your example would not be considered legal as stated.
The fuel minimum requirements would most likely preclude a scenario where a fuel stop was actually required after 40 miles when the next point of landing is only 40 miles further. It's theoretically possible of course, depending on the fuel state at the first departure, but highly improbable. So, lacking a clearly compelling reason to justify the fuel stop after 40 miles, the stop would be viewed as a transparent attempt to avoid the rule.
Now, all that said, that's not to say there's no chance you couldn't get away with it:

The rule is, presumably, designed to ensure the safety of the passengers by preventing the pilot from having to navigate beyond their training in potentially deleterious conditions (i.e. where the weather is less certain). "Resetting" the navigation with a landing after 40 miles does seem consistent with achieving this goal. If the commercial pilot can be considered competent for any flight 50 NM or less, then surely they are competent to complete two flight legs of that distance, such that they are always starting from a clearly-defined reference point (i.e. the airport where they landed).
A much more probably-legal scenario would be where there are different passengers for each leg, at least in part. In that case, then there's a clear justification for the interruption of the flight just short of the 50 NM limit. Indeed, one could try to use this scenario as justification for any other "break the flight into multiple legs" scenario. After all, if this more compelling scenario is legal, it's hard to see how other scenarios that are operationally identical regardless of the specifics would be not legal.

All that said, it behooves a commercial pilot to behave cautiously and always with both the safety of their passenger and compliance with the regulations, two things not always directly in correlation with each other. Given the FAA's power to suspend or even revoke commercial privileges, and given the relative lack of recourse by the pilot to reverse decisions like that (it happens, but rarely), the best thing to do before attempting this sort of thing would be to contact the local FSDO and just ask them.
With an official opinion in hand, one will know for sure how the authorities who might or might not cite a pilot under that regulation will actually treat such flights.


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How can I calculate the tail area required to achieve longitudinal static stability?
I've got a longitudinal stability question. I've not attempted this question yet as I didn't know where to start from, also my lecturer hasn't explained static stability to us clearly and I needed a step by step if that's fine.
These are the parameters (its a 747)

And this is the question:

The design requirement is that the aircraft should be stable when the
c.g. is less than 1.8 meters behind the aerodynamic center of central
wing.
What is the minimum required tail surface area to achieve this? Tip:
first derivate the expression for longitudinal stability.

Your previous questions have already established that your professor oversimplifies things and has an issue with teaching basic concepts.
That lets me assume that the aerodynamic center of the wing-body combination is the same as the aerodynamic center (a.c.) of the central wing (which is not true, but never mind). Otherwise this question cannot be solved.
Static stability means that a change in angle of attack (AoA) changes the aerodynamic moments around the center of gravity (c.g.) such that a restoring moment change remains. Mathematically speaking this means: If the derivative of the moments over the angle of attack is negative, the airplane is stable. Here we look for the minimum tail area that creates stability, so the desired derivative should be zero, becoming more negative with larger tail areas.
Now let's get to work: What are the contributions to the aerodynamic moment change over AoA?

Wing: $\frac{\delta M_{Wing}}{\delta\alpha} = 1.8\cdot0.098\cdot511\cdot q$

Since neither speed nor altitude are known, I cannot use a number for the dynamic pressure and keep its symbol ($q$). All other parameters are given.

Tail: $\frac{\delta M_H}{\delta\alpha_H} = -29.7\cdot0.047\cdot S_H\cdot q$

The 29.7 meters is what remains between the c.g. and the a.c. of the tail. But we forgot one thing: There is downwash to consider! When the real AoA change is 10°, it is only 9° at the tail. Mathematically, this means $\delta\alpha_H = 0.9\cdot\delta\alpha$
Now it is time to set the sum of both moment derivatives equal to zero:
$$1.8\cdot0.098\cdot511\cdot q\;+\;0.9\cdot-29.7\cdot0.047\cdot S_H\cdot q = 0$$
Mercifully, the dynamic pressure can be cancelled in both terms (another such simplification which won't survive contact with reality …) and we isolate the tail surface area on one side:
$$S_H = \frac{1.8\cdot0.098\cdot511}{0.9\cdot29.7\cdot0.047} = 71.75\:\text{m²}$$
which is 14% of the wing area and far less than the real number of 136.6 m². Umer, you should now at least check the units in all those equations to see whether they make sense. Report in the comments what you have found out!
The real number is higher since the rear c.g. location does not produce an indifferent longitudinal stability but leaves a nonzero minimum. Unfortunately, this minimum is not given. Another reason is that the tail is normally sized for the landing case with full flaps. A third reason is that you want to keep some margin for maneuvering - again, such a margin is not given here.

That first paragraph. Really?

@Koyovis Really! This guy has asked three questions, all of which showed very limited understanding on the side of his/her professor. The prof should at least stick to consistent wording and mention important effects, even if they are not part of the solution. But students should be aware of the simplifications!

I'll leave it for @Peter Kampf to explain the math, but I find it's easier to conceptualize static stability if you simply think of the airplane as a weathervane turned so its pivot axis is horizontal.  Static stability simply is the forces making it point into wind, except the wind is changing in a vertical direction rather than a horizontal one.
Instead of the weathervane's pintle, the pivot is the aircraft's center of gravity.  The "vane" of the weathervane, the part the wind is blowing on to align it, is the aerodynamic center of the horizontal footprint of the entire aircraft, the sum of all of the dynamic forces acting in a vertical up or down direction on the fuselage, engine nacelles, wings and tail.  This is the Neutral Point.
The Neutral Point has to be behind the pivot, the Center of Gravity. Any change to the configuration that changes the aerodynamic horizontal footprint shifts the Neutral Point. Going back to the weathervane, if you tape a piece of cardboard to the back of a weathervane, you've moved its Neutral Point aft and it wants to point into wind harder.  It's more stable. Tape a piece of cardboard forward, and you move the Neutral Point forward, reducing its stability or pointing tendency.  Tape a big enough piece so there's equal surface area on each end, and you move the Neutral Point to the pivot axis and the weathervane no longer points but just wanders any which way. Its stability is neutral. Tape an even bigger piece, and the vane wants to switch ends until it finds stability going the wrong way.  It's unstable.
So the question is basically how big does the tail have to be so that the overall aerodynamic center, the Neutral Point, of the entire airplane is sufficiently aft of the Center of Gravity for an adequate pitch axis "weathervaning" tendency, or positive static stability. Making the tail bigger moves it aft, making it smaller moves it forward.  The formula for longitudinal stability derives the surface area required of the tail when its influence is combined with the aerodynamic forces acting on the overall horizontal footprint of the entire body.
This is what got Boeing into trouble with the MAX.  The engine change, moving the nacelles forward, basically shifted the aerodynamic center of the weathervane too close to the pivot axis (the most aft C of G), reducing its "pointing ability" in certain flight regimes. As if you'd added surface area to the front end of your weathervane.
Beyond that, you then get into trimming forces, or the use of opposing pitching moments between tail and main wing to create force balances that allow the aircraft to point "off wind" (vertically), which gives you the ability to maneuver and be stable at various angles (angles of attack on other words, which you need to do to make the wings lift in the first place) other than directly into wind, as well as have good dynamic restoration characteristics.  Without trimming forces, the airplane becomes a very statically stable lawn dart.

Thanks, now I feel pressured to write out the answer!

But what is the tail surface area, as asked in the question?

Beats me.  That's up to Peter! lol.  I was just providing an analogy framework to help him out with understanding the general concept of static stability, which the OP said was poorly explained.


text:
Do airline companies have a Chief Medical Officer?
I was wondering if airline companies have a Chief Medical Officer who is in charge of the well-being of passengers. For example:

in timing the lights on/off moments on intercontinental flights;
timing and type of food served, to support periods of sleep, wake and disembarcation;
airconditioner procedures, to ensure passenges don't get sick from too cold airconditioners (for example when the captain released power from the engines to the airconditioners after take-off, while the cabin crew had set it to min-min on an insufficient EPU at a hot airfield, it happily started blowing 10 degrees C in my neck);
methods and moments of desinfecting the air/passengers when contagious diseases break-out (or a flu epidemic in the US, with all its internal air-traffel);

This may be more of a Travel question.

I think it counts, it's about airline operations and we usually answer those, don't we?

@JayCarr it seems like it's not necessarily off-topic here, but it also seems like not many people here could answer this, and there might be more at Travel, though I'm not sure.

@raptortech97 We have several airline pilots who frequent this stack and could possibly know the answer...  I'd venture to guess we have more airline employees here than Travel does.  Just my thoughts though..

@JayCarr ah, that actually makes a lot of sense

I'm not sure how much any of your things would have to have an airline doctor worrying about them -- lights turning on/off I'd expect is basically "turn off when it's night, turn on when it's day," food served I'd expect to be the province of a chef and logistics people, air conditioners should be set to room temperature, contagious disease is the job of governments to handle.

@cpast: the light intensity and color influence the brainrithm and can (should!) be used to induce a sleep period in a manner that, once the ocean crossed, the passenger wakes up having slept well. The food must be easy to digest because the passenger sits all the way. In the morning it must ad energy to prepare for disembarkation, etc. Temperature must likewise be set (and not for the comfort of the cabin crew, as happens!). It is not simply "lights on/off"!

I'm not sure if every airline is the same. However each major airline I am familiar with does have a medical unit, usually headed by a doctor.
However, most of their efforts are actually dedicated internally - by that I mean looking after their employees. Things like:

Assessing the employees ability to return to work after an injury or illness
Fatigue management policy
Assessing whether a certain condition is acceptable to work with
Overseeing drug and alcohol rehabilitation (in some cases in Australia at least the employee is not automatically fired for being drunk/on drugs, they are allowed to complete a program and continue work)
Occupational health and safety assessments
General 'doctoring' duties for aircrew

Of course they are also involved in passenger health policy development, but as I said - the majority of their work is dealing with their own staff (at least, this is from my viewpoint, but I may not be seeing the whole picture).
In terms of an outbreak like you have mentioned, there is always an action plan ready to be put in place which is developed with the government health department (or CDC in USA).

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text:
Does maximum altitude increase when pilot is acclimatised?
Normally 10,000 ft is considered safe altitude for emergency descents and 12,000 ft a maximum that can be flown in unpressurised aircraft without using supplementary oxygen.
But when flying in the mountains with a crew that normally lives at say 5,000 ft or more, can they afford higher altitude? And by how much?
Note: The 12,000 ft I've seen somewhere, but I don't know whether it is actually a regulation or just recommendation.

I've gone flying after spending a week skiing between 10k and 12k feet and I certainly felt like I could spend all day at 15k without O2.

Is there a legal limit to fly unpressurized aircraft?

@vasin1987 yes, not to fly, but to fly without supplementary oxygen.

@falstro yeah that would be a good answer here. However i guess OP expect the answer where no supplementary oxygen is requireed. Ie can pilot get used to high altitude that lack of oxegen is not a problem. Hmmm i wonder what answer this question gonna get :)

Yes, maximum possible altitude at which a pilot can fly an Unpressurized Airplane without the need for supplementary oxygen does increase with acclimatisation. But the international aviation laws do not take this into account and hence they have a general altitude limit for unpressurized flying without supplementary oxygen.

Do you have a reference for what that limit is?

@JanHudec Not sure about ICAO's model regs, but for the US it's FAR 91.211: Supplemental oxygen is required for crew if you're above 12,500 feet for more than 30 minutes, or if cabin altitude is above 14,000 feet for any length of time. Above 15,000 feet it's required for everyone onboard. (There are other requirements that come into play if the aircraft is pressurized.)

Since the question talks about emergencies and mountain flying, I think it'd be appropriate to add that in emergencies, rules come second while safety comes first (as described in FAR/AIM §91.3(b)).

@Maverick283: Actually, no, the question does not talk about emergencies.

@janhudec alright, it is talking about a "emergency descent"... I'd still count that as an emergency though?!

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text:
Why don't small planes use turbine (turboprop) engines?
Turboprop engines are more fuel efficient1, lighter for the same power, mechanically simpler and consequently more reliable.  They are also slightly easier to operate (no need to fiddle with mixture) and burn cheaper fuel. As a result they replaced piston engines in all transport aircraft long ago. So why didn't they find their way into small GA planes (basically everything above 8-10 seats uses turboprops, but in the 4-seat category, nothing does)?
1Actually looks like I was mistaken. Reciprocating engines seem to be more efficient.

Similar Question.

If I remember correctly my lectures on propulsion, turbines are more efficient than piston at higher altitude and thus not that much more interesting for small GA planes...

@LudovicC. I thought the high altitude was the jet vs. propeller part (not turbine vs. piston), and the reason why many modern airliners have turbofan rather than pure jet engines. I believe a lot of helicopters use turbine engines, and I think they're quite happy flying low, or am I missing something?

@LudovicC.: That's not efficiency (energy delivered per unit of fuel) but power. Maximal power declines slower with altitude for turbine engines. But since it also declines for propeller, turboprops are usually limited to about FL250 anyway (while turbojets/turbofans happily operate above FL400). But turbines should be more efficient (burn less fuel for the power they are delivering) at all altitudes.

@Jan Hudec: Don't tell that to the crews of airplanes like the Tu-95! Turboprops can fly as high as subsonic jets if they are designed properly. The power over altitude behavior is the same, only at high Mach numbers the props put an earlier limit to the maximum speed. Think of a turboprop as a jet with a large, geared, unducted fan.

@PeterKämpf: Of course they can. Just the propellers get impractically huge (as Tu-95 has), because the power per unit of propeller disc area is limited at altitude. The ducted fans have other ways to improve the efficiency and also the core exhaust contribution becomes more significant at higher speed.

Related question:  Why are propeller airplanes still made?

The PC-12 has a certified ceiling of 30,000ft, but is limited in the US to FL250 ft because of supplemental oxygen requirements (http://www.flightsimaviation.com/data/FARS/part_91-211.html). May be true for other turbopropos as well.

Helis use turboprops for 2 reasons: first, because they are more reliable, and second because of the power-to-weight ratio. The Allison 250 is very popular in the smaller copters http://en.wikipedia.org/wiki/Allison_Model_250 with a power-to-weight ratio of 1.838 hp/lb. My TIO-540 has a power-to-weight ratio of 0.68 hp/lb. Since costs aren't as much of an overall factor for helis (versus GA airplanes), they can use turbines.

It would be possible to argue that turboprops are more "efficient" than pistons in terms of power-to-weight rather than fuel burn.

First, piston engines are more efficient than turboprops, so their operational cost are lower. This also means that the system mass (engine plus fuel) for a trip is lower once you go beyond small ranges. In a helicopter, the engine mass is more important, because average flight times are much shorter, so you find many helicopters with turbo engines and only few with pistons.
Next, there is a well developed infrastructure for maintenance on those pistons, and new turboprops would need new, expensive infrastructure (trained people, tools, spare parts ...)
And then there is simply no incentive to develop a new GA engine. The expense to get it certified is too large for the rather small market. This is the same as for Diesel engines.

Efficient in general, or efficient at lower altitudes?

I don't think the development argument applies here. Back when the current engines and aircraft were developed, turbines already dominated the higher power market.

... but of course the first argument is enough. It seems reciprocating engines are indeed more efficient. (Diesel engines are even more efficient than spark-ignited ones, but they are also quite a bit heavier).

@egid: Efficient in general, if a sufficient turbocharger is fitted. The more the engine is charged, however, the more the thermodynamic cycles of piston and turbo engines converge.

@Jan Hudec: Turbo engines make more sense in the higher power settings simply because the airplanes with more powerful engines fly faster. With a piston engine, the power is constant over airspeed, so thrust goes with 1/v. Turbo machines can make better use of the kinetic energy of the airflow, so their thrust drops less with speed. Once you want to fly at Mach 0.6 or faster, piston engines don't cut it anymore. At lower speeds, turbines never were a real option, except for helicopters.

@PeterKämpf: Are you sure the reverse flow turboprops (the most common kind, where the air is turned 180 degrees and flows forward through the burner so the power turbine is at the front and has shorter shaft) make significant use of the airflow energy? (Of course turbojets do, but that's a different story)

@Jan Hudec: The internal flow is less important, here the intake design is what counts. How much of the kinetic energy can be transformed into pressure? And that is less a question of engine design than of engine integration into the airframe. Bending the internal flow path will incur some pressure loss, but you get a compact engine with less torque on the main shaft. And bending the exit flow again will give you most of the possible thrust gain from accelerating the flow. But there is a reason why reverse flow is used only on small, relatively low-speed designs.

Also, reverse flow engines have a "built-in" inertial separator, which removes ice and FOD from the intake air. http://i601.photobucket.com/albums/tt96/pivo11/neds%20stuff/tp1.jpg

I realize this is a heli-oriented comment, but I'm addressing some heli comments made above. Most helis (including the smallest, like the MD500 family) have turbines because they are much more reliable and easier to operate in the single-engine configuration than a single-engine piston heli. Piston helis are used solely because they're cheaper, especially for training and Part 119 commercial ops. And in third world countries, turbine helis are used because avgas is nearly impossible to get, but JetA is plentiful.

The same in response # 5 may apply to Wankel Rotary Combustion Engines and Single Sleeve-Valve reciprocating engines for General Aviation, as of today, nobody can afford the certification expenses

Small turboprops are terribly inefficient, sometimes burning as much as double the fuel per HP/Hr  as a similar power piston engine.  Even a half century old engine, like an O-540, can get 0.42lb/Hp/Hr specific fuel consumption, while an Allison/Rolls, C250 turbine will burn 0.8Lb/Hp/Hr in cruise (Turbines get the lowest specific fuel consumption at max power).  Very large turboprops, like the Allison/Rolls 2100 can get as low as 0.4Lb/Hp/Hr but only at max power.

This is probably due to certification inertia, at least in some part:
A Piper Archer is certificated with a specific engine type (If you scroll down to page 21 of the TCDS you'll see it's a "Lycoming O-360-A4M"). Piper can essentially make as many Archers as they want under their production certificate as long as they conform the type certificate.
Changing the engine is a major change to the type certificate, and would require more certification test flights, paperwork, and ultimately money from the manufacturer, and sales would be uncertain (because it would be different -- A flight school flying 50 planes that are all powered by Lycoming O-360s is not going to want to take on one turbine-powered plane, it would be a maintenance headache).

It's worth noting that for some portion of the avgas-burning GA fleet efficiency gains similar to (or better than) turboprops can be achieved by converting from old-style Lycoming and Continental engines (which are effectively 1960s engine technology at best) to engines like the Rotax 912 with modern electronic ignition and engine management systems. An 80HP Rotax would pull a Piper Cub along just as well as an 80HP Continental - The barrier is again in certification (and cost to modify existing aircraft).

On the technical end of things, a turbine engine is overkill for most small GA planes. We're talking about aircraft that usually require 100-300 horsepower (and if we're considering light GA training aircraft you're usually below 200 horsepower). The popular PT6 turboprop engine starts around 500 shaft horsepower and goes up from there.
Also while a turbine engine is a relatively simple device a turboprop gets to be pretty complex -- The turbine section is still spinning at turbine speeds, and while this isn't a problem for a jet engine on a turboprop it could easily make the propeller tips reach supersonic speeds (the "Loud and Inefficient" operating range). This necessitates a reduction gearbox to produce an appropriate shaft speed, which in turn necessitates maintenance of the gearbox.

No, I don't buy the argument about certification. If that was the case, the newer designs would be certified with turbines from the start. And "newer" does not have to be all that new, since turbines dominate the higher power market at least 60 years. Or we'd at least see some conversions. But there are none, so there has to be a technical reason why turbines with less than ~150-200 kW don't exist. And it's not just gearbox. Most reciprocating engines are geared as well.

@JanHudec You can "buy" whatever argument you like - In reality it's probably the confluence of several :-) Specifically Re: certification though, the simple fact is certificating a major aircraft component (like an engine) is an involved process: Irrespective of any possible technical impediments, a new 200-hp turboprop engine would still need to go through the FAR 33 (engine certification) process in order to be used in a certificated aircraft, even a "clean-sheet" design built for that engine. That's lots of money and time for an uncertain return on investment for the engine company...

The engine currently in use had to go through that certification process and it did so when turbine engines already dominated the higher power market. So back then a decision was made to stick with reciprocating engine for low power uses and that decision was not affected by certification because either variant had to be certified.

@JanHudec most, if not all, of the powerful GA piston engines (large 4- and 6-cylinder designs), are developments of engines that have been around since the 1940s or 50s. There is no way that turbine engines dominated that time period.

Aside from the certification costs, the design costs should not go unnoted. Designing a turbine engine (or any engine, really) is not a simple matter. Making iterations on existing reciprocating designs is a lot easier than starting mostly from scratch with a turbine. Designing and fabricating turbine disks that won't fly apart is easier said than done.

First, for a given power rating, a turbine engine will be a lot lighter than a piston engine. Think about what that does to the aircraft's balance: the nose gets a LOT longer, not to accommodate the hardware put to put the weight where it needs to be. Of course, this only applies to conversions.
Second, a turbine engine will cost a lot more than a piston engine. Smaller turbines cost more per unit of power than large ones, so overall you will be paying more for the same result. The engine might use less fuel over it's lifetime but you still have to pay for it up front.
Third, while turbines in general require less maintenance, when something does have a problem the bill is not for the fainthearted. And you can cook a turbine pretty easily.
So in total it just comes down to money. If you really want one, Soloy sells a conversion kit for a Cessna 206 that will put a 450hp Rolls-Royce turbine in the nose and make you the envy of every other light aircraft owner in the region. The kit starts at $275,000. Plus engine. So you are looking at over half a million to modify an airframe that costs a bit less than that. If you have that much spare cash, pick up a matching MTT Superbike with an only slightly smaller turbine. As Jay Leno said, it's "the best ever motorcycle for shutting up the Harley guys."

Well, some do. Meet the Marchetti SF.260TP:

This is a very popular light single for aerobatics, which has also seen military use in CAS and observation roles. If you're a Bond fan, you'll know about this one; it appeared in Quantum of Solace. Other turboprops in the small single-engine class include:

Piper Meridian
One Aviation Kestrel
Pilatus PC-9, PC-12, PC-21
Cessna 208 Caravan/Super Cargomaster
Epic EW1000, LT
Socata TBM850, TBM900
Grob G-120TP

As for why not all small singles are turboprops, the other answers have it; turboprops are less fuel efficient at the low altitudes small single typically fly at (they're great for higher altitudes) and they're a different beast to maintain. They're also more expensive; the Marchetti is about a $1.6 million dollar plane, 3 times the cost of a Piper Archer 235, though not all of that is the engine.

One more point regarding cost: most of us are probably flying planes built in the 1960s, if not earlier.  My Cherokee 180, built in 1966, could be purchased today for about 2% of the cost of that Marchetti.

Turbine engines are less fuel efficient at all altitudes.  Smaller turboprops, like the PT-6 and TPE-331 will typically get cruise specific fuel consumption lbs of fuel per Hp per Hr of between 0.6 and 0.7, while a piston aircraft engine burns between 0.4 and 0.5.  Some new diesel aircraft engines burn as little as 0.35 lb/Hp/Hr.  A really small gas turbine like the Allison Rolls 250-C20 (420 hp) will burn anywhere from 0.6 -0.9 Lb/Hp/Lb.  Turbines are most efficient when running at 100% power, so they turn in their best fuel mileage at high altitude.

Another thing to remember about the specific fuel consumption of a turbine engine is that it is HIGHLY dependent on the size of the engine. Small gas turbines (of the sort that may be used in small private aircraft) are inherently inefficient because they cannot run the kinds of pressure ratios required for high efficiency. For instance, a 2180HP PW119B has a pressure ratio of 13.2:1 and a SFC of .49. Step that up to an 11000HP Europrop TP400-D6, and pressure ratio climbs to 25:1. SFC drops to .39. And the 56300HP GE LM6000 (a marine and power generation turboshaft based on the CF6 turbofan) has a pressure ratio of around 32:1 and a SFC of .32. Because of this, piston engines have a fuel efficiency advantage at low power outputs, but a fuel efficiency DISadvantage at high power outputs. Piston engines therefore are favored in small aircraft for this reason and initial cost. But turbines make MUCH more sense in larger aircraft due to both their higher efficiency than large piston engines and reduced maintenance costs.
Someone mentioned helicopters here. And helicopters are a different animal altogether. A basic characteristic of helicopters is the fact that they require LOTS of power on a continuous basis. This would take a HUGE toll on piston engines in terms of reliability (think of how long a car engine would last if you constantly drove it at full throttle), but is not an issue for a turbine. Turbines are also smaller and lighter (ever seen a Sikorsky Mojave with its ridiculously huge piston engines)? All of these factors make turbines MUCH more practical than piston engines in helicopters.

Piston aircraft engine are typically run at 75% power at cruise, and 100% power during takeoff. Car engines definitely could not handle this kind of load, but aircraft engines are built for it.

I'd say choice between a turboprop or a reciprocating engine may depend on your power needs, and the type of 'mission' the engine/aircraft will endure.
Turbines and Turboprops, even with its advantage of weight to power ratio, are not that good regarding SFC, if you check this in www-jet-engine.net civil turboshaft-turboprop specifications, most of it have an SFC in the range of 600 lb/shp/hr, or more.
The Harry Ricardo and Roy Fedden Single Sleeve-Valve distribution Aircraft Engines provided the best ever SFC in a gasoline aircraft engine: 0.42 lb/BHP/hr in an Hercules Centaurus, 3272 cu. in., with a weight of 2695 lb, for a power of 3150 HP. Even more impressive results were obtained with an Open-Sleeve, acting as an annular piston with 10% of piston area, transmitting 3% of power, in an experimental 2-Stroke Compression Ignition Engine by Harry Ricardo. No Sleeve-Valve engine is produced today, but new replacement sleeves for old engines are still manufactured, they said this in AEHS, but I failed in locating the seller of Sleeves for old SSV engines.
Best SFC ever seems being obtained in an open-sleeve valve, two-stroke, compression ignition engine, by Harry Ricardo, at 0.34 lb/HP/hr ('The high speed internal combustion engine', 1968 ed)
A poppet valve engine of the times of Bristol Hercules, the Wright R-3350, 2907 lb weight, also air-cooled, with same number of 18 cylinders; 3348 cu. in.; 2800 HP, used 0.72 lb/HP/hr, but this was greatly improved in the Turbo-Compound units, that added 500 lb weight more.
The Sleeve-Valve engines were known for its reliability and low wear, the no-lubrication zones of TDC and BDC are wiped out by the continuous sleeve movement, the Bristol Hercules had a TBO of 3000 hr or more, for 2000 hr TBO in the Wright cited (data in AEHS and other sources). Sleeve-valve engines, having no hot spots in combustion chamber, can work on low octane fuel at higher Compression Ratios than poppet valve engines.
The experimental Single-Cylinder, 500 cc, Sleeve-Valve gasoline unit built by Mike Hewland for Automobile use was reported having an SFC of 0.45 lb/HP/hr in the racing version, and 0.39 lb/HP/hr in the economical version (Car&Driver, July 1974), working even with creosote.
A Rolls-Royce Turboprop, the Dart RDa 10.1 series, with 2915 HP and a weight of 1207 lb, consumed 0.550 lb/HP/hr.
You can't find today reciprocating engines in power ranges above a certain limit.
The Wankel Rotary Combustion Engine, that had extensive development for aviation and other uses by Curtiss-Wright, John Deere, NASA, NSU-Citroën, Sachs, Aixro, Rolls-Royce, Mazda, had an SFC of 0.46 lb/HP/hr with a weight of around 210 lb for a power output of some 120 HP, street car version, as reported in an early Mazda NA Wankel Engine, 2 Rotor, liquid cooled, a Wankel will work with no toil with unleaded gasolines around 80-90 ON, also with 10% Gasohol, that provides lower SFC and reduced operation temperatures and thermal load, some added lubricating oil to fuel and working chambers is always used in Wankels, no need of changing lubricating oil.
A major breakthrough was achieved recently in the Florida University, where they proved that adding: 'Heat Pipes', for cooling the housing and side plates of a formerly air-cooled housing charge cooled rotor UAV UEL engine reduced maximum temperature to a mere 129º C, and maximum temperature difference between engine parts to 18º C, the use of: 'Heat Pipes' for cooling Wankel RCEs, as it were used in satellites, may be an enormous step forward in Rotary Combustion Engines, as it eliminates most if not all thermal dilatation differences between parts of engine, making design and construction much easier, also improving reliability, from a reduced wear, enhancing power, reducing emissions and boosting fuel economy. Who could ask for anything more?
Standards for reciprocating aviation engines were around 1 kg/HP weight to power ratio, and 250 gr/HP/hr SFC.
No Wankel Rotary Combustion Engine seems having obtained yet an FAA certification for regular general aviation use, even when it are considered safer than reciprocating engines, Wankel RCEs failures tend not being total and instant as in reciprocating engines, a residual temporary lower power working would allow for more safe landing opportunities, this safety concern was approached by something close to the basic concepts in an hybrid car by Axteraerospace.com
This one is an example of Turbine for General Aviation build in Argentina, basic arrangement reminds the first Turbine by Hans Joachim Pabst von Ohain

Fortunately this stereotype is changing, albeit slowly. Small GA engine development has been stuck in the back seat for generations and it's great to see a number of new start ups making the push towards change. The TP100 Turboprop by PBS is once such product. I really hope they reach their potential and bring the product to market, even if only to experimental parties. Continental is aggressively developing their new diesels and will be launching a new 230 ish hp unit in the next year. It is already in testing in China. If they get that right, it will be a Lycoming killer. They've already begun conversions with the 155 hp version in Cessnas.
One reason the lower power (150-250 hp) turbines have not been developed more rapidly is that GA pilots have not pushed for them. They know they take more fuel, even if the power/weight ratio is far superior. The operating costs make them hide in the corner.
RR/Allison made an effort to refresh the aging 250 with a modern turbine that would appeal to the Cirrus crowd but bowed out when the 2008 financial crisis hit. The problem with companies like Allison is their development costs are through the roof.
Change in this space is going to require guts from the little ducks of the world. Many of which are now starting to appear on the scene. There is a well know shortage of 100LL around the world and soon here in the US. That and the high price of carting this boutique (not shipped in pipelines) fuel will fuel the market for these new products.
There is no rocket science at work here. It's old technology (albeit with a bit of electronic intervention) that just needs a market to grow in. Today we are moving towards that market. Get used to it and consider tossing that ole' Lyc.

When Ford and GM experimented with automobile turbines, fuel economy was extremely poor, and tooling costs the same, wothout commenting on exhaust emissions

It's not just fuel economy and emissions that made car manufacturers shy away from turbines. The rapidly fluctuating power requirement in road vehicles seems to have been an even stronger reason to stick to lower inertia engines. When the traffic light turns green you don't want to wait until your Ford has spooled up!

Actually they do have a number of smaller turbine powered aircraft these days.  And turbine STC conversion kits for existing GA aircraft are popular - the P210 Riley Rocket quickly comes to mind as does the PA-46 JetProp plus high end experimental aircraft such as the Lancair Evolution and IV-P Turbine.  These turbine conversions give all the advantages listed above, but at a higher fuel burn.
The primary deterrence against an entire turbine GA market are the higher operating costs and excessive performance for a neophyte or casual flyer.

The cost impact of a turboprop has been underestimated in many of the earlier answers.
Turbine rotors are subject to very high temperatures while turning at very high rpm; the G-force on a turbine blade is enough to make your eyes water. Due to a minute manufacturing fault or operational error or metal fatigue, one blade lets go; then it will tear through the steel engine case, through any unlucky oil or fuel plumbing, then through the aluminum cowl. The manufacturer will spend hundreds of thousands of dollars to prevent this from happening, and must recover that cost from engine sales.
And that's just from one broken blade, weighing maybe a few ounces.
If a much-heavier rotor disc breaks from fatigue, it could saw the airplane in two.
So yes, a turbine burns cheaper fuel, but more gallons than a recip. You can see there are pros and cons!

The cheaper fuel can also be used in a diesel and more efficiently.

Matt got a lot of it right.
Dynamic compressors and turbines tend to get less efficient as their sizes decrease, because of Reynolds' number effects and because of limits on materials and manufacturing.  To get a good stage pressure ratio in a compressor, a tip speed of at least 300 m/s is required;  exceeding 500 m/s is not unheard of.  Since the centripetal acceleration is inversely proportional to radius at constant tip speed, centripetal loads can be too high for most materials.  Tip clearances have to be small -- about 1% of tip chord -- for best efficiency, high temperatures demand cooled blades and vanes in the hot end, and that means that blades and vanes have to have cooling passages inside the blade.  This is easier in a PW4084 than an Allison 250, as the PW's blades are a lot bigger.

It is much more expensive to build a turbine engine than a piston engine. Turbine engines can be built that have a longer TBO than a piston engine, partially offsetting that cost, but that matters more to users who have high aircraft utilization per year. One tends to see turbine engines used in commercial operations, 1000s of hours per year, and pistons in private operation 100s of hours per year. As engines get larger and more powerful, piston engines get more expensive and complex because they are limited by engine knock. So turbines dominate at larger sizes for all users. It is all driven by dollars. No bucks, no Buck Rodgers.

Care to share some specifics (including references) on how engine knock limits piston engine size? After all, these guys have managed over 107,000 HP out of a piston engine...

About the questions of knock limiting the size of piston engines and 107000HP marine diesels, knock is not an issue. Rather, the limitation on the size of piston engines is PHYSICAL SIZE. Reliability is also an issue as the number of reciprocating moving parts skyrockets. But size is what REALLY kills the deal when it comes to piston engines. ln my other post, I mentioned the Sikorsky Mojave. That helicopter looks downright bizarre because of the large pods protruding from each side of the fuselage. Those pods each contain a 2100HP Pratt and Whitney R-2800. By comparison, the 5000HP Honeywell T-55s on a Chinook look almost like toys. If you wanted to power a Boeing 747 with piston engines, each engine would probably have to be the size of a decent sized plane in order to produce the ~55000-60000HP that each CF-6 produces (consider how big the R-4360, the largest production aircraft piston engine was, and it topped out at around 4000HP). The 107000HP marine engine is an extreme example of this. These engines are literally called 'cathedral engines', and live up to that name with 38 inch bores. Yes, it may produce about as much power as two CF-6s. But these engines are as large as they sound, and not exactly aircraft material. On the other hand, size is not so much an issue in an ocean going ship. And reliability is not too bad for these engines since the large size means that a smaller number of super large components can be used.

For me, the answer is money. A P&W PT-6 turbine engine is going to run you between \$500,000 USD and \$750,000 USD to be overhauled. A turbocharged piston engine, even a very expensive one, isn't going to cost more than about \$50,000 USD to overhaul. Other than that, for certain missions such as over-water/mountain operations I'd take the fuel burn for reliability and weight savings trade-off in a small plane, though you can expect to burn 2 to 3 times more fuel in a small turbine vs a large piston. Note that there are many piston types that have STCs available to replace the standard piston engine with an aftermarket piston, but as I mentioned the costs are relatively astronomical to do so. For example, the Silver Eagle Cessna 210. http://www.onaircraft.com/the-planes/the-silver-eagle-ii/

Just to be clear.  Though it is true that small turbines are much less efficient than small aircraft piston engines, the difference is not 2-3 times.  It may well be true that a Pilatus PC-12 burns 2-3 times as much fuel as a Cessna 210 or Piper Malibu, that Pilatus is a much bigger, faster, plane that carries much more load.  A Cessna 210 with a turbine engine instead of the piston engine, does burn more fuel at the same speed but only about 30% more for example.

Small aircraft dont need the only advantage turboprop allows: more power to weight ratio.
A turboprop is more combersome than a piston engine for lower power to weight ratios
It will not save so much weight comparing to a small displacement piston engine since a lot of the weight comprises other parts of the propulsion system beside the engine ( gears, fuel systems, cooling, charging system, propeler, etc), and a turbine allways consumes more fuel.


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Why is an outward spinning propeller better in theory on the North American F-82 Twin Mustang?
In this question the Wikipedia article of the F-82 Twin Mustang is quoted. In the third paragraph of the article it says:

In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control.

There was a comment in the other question that questioned this statement:

according to that article, an outward-spinning engine configuration is better in theory. i don't see how this would be. in a single-engine
  scenario, wouldn't the working engine torque push the non-working
  engine side down?

A spinning propeller should also produce more lift on the wing section behind it as it is increasing the airflow over that section. Forgetting about the torque for a second, this would already push the other wing (with the failed engine) down.
[EDIT: As answered in the comments, the lift generally has way more effect then the torque.]
So what exactly makes it more favorable to have a outward-spinning configuration in theory?
Also: Does this only apply to this aircraft, or is it valid in general?
Why does it say that the outward-spinning engine configuration is better in theory? What am I missing?

This is actually answered in the answer to the question. The vortex generated by the propeller increases lift on the upswing side because the angle of attack is effectively increased. Because lift is generally larger than thrust and thrust larger than torque, this change is generally greater than the torque. YMMV, though.

@Ville_Niemi Okay, so if I got that right you are saying that it doesn't really matter if there is any more torque as the lift has much more effect anyway. But why is an outward-spinning engine config better in theory then? O.o
Thanks though, will edit the question a bit...

the more i researched the other question, the more i think this "theory" is bunk.

@erich Jupp, I really just want to double check that here. Would be interesting to get a more in depth explanation about this sentence as well

also relevant to ask: better in theory in general?  or better in theory for this particular design?

The theory is actually quite simple and straightforward. It should go like this:
In one-engine flight, the asymmetric prop position will pull the aircraft into a sideslip. The sideslip-induced rolling moment will then proceed to roll the aircraft such that the airplane rolls over the dead engine - something every multi-engine pilot learns to avoid early, because it will be hard to get the airplane back to level flight in this configuration.
If the single operating propeller will give the center wing an increased angle of attack due to the propeller swirl, and the outer wing a lower angle of attack, the prop produces a rolling moment which works against the sideslip-induced rolling moment, and should make it easier to avoid rolling over the dead engine. Engine torque will work against this moment, but the induced rolling moment of the wing is stronger, leaving a net moment which rolls the aircraft over the operating engine.
This should apply to all multi-engined aircraft. My reasoning why it didn't work for the F-82 can be read here.

Honestly, I never cared about the minuscule amount of lift generated on the wing by the turning propeller. It's not the lift you need to worry about; it's the angle of attack. When I flew multi-engine prop planes, there was a reason that one of the engines was considered the critical engine (ie the one that most affects the handling of the aircraft) A descending propeller blade creates more thrust than an ascending one (p-factor). And since you want the thrust to be as close to the front or back of the center of gravity as possible to prevent yawing around the CG, you'd prefer to have that descending blade as close to the middle of the aircraft as possible. Torque can play a part with a big engine (like the old Merlins), but the p-factor usually tends to be the bigger issue. Peter is right, you do tend to roll into the failed engine, which can be pretty bad if you don't or worse can't stop it (angle of attack again). You're already doing bad things to the aerodynamics of the aircraft operating on one engine.
I don't know how the aerodynamics of this specific aircraft were, but asymmetrical thrust is asymmetrical thrust no matter the aircraft. The last thing you want to do is stall like that.

combined with peter's answer, the aircraft will both roll and yaw in the direction of the dead engine.  in your opinion, the yaw is the more adverse factor?

The yaw is the one that's going to hurt you. The roll is what will make the aircraft stall so that the yaw hurts you. The yaw will usually be more pronounced, but you'll also want to address the roll before it becomes too pronounced. I always taught that you provide a couple of degrees of bank into the operating engine to try to offset the roll.

To clarify just a little bit more, bank increases the load factor on the aircraft and increases the angle of attack (thus the stall speed) of the the high wing. Yaw is what turns a stall into a spin. Granted, you're stalling the wing that's still got the operating engine, but you're probably not far from stalling the other wing too. Plus, again with the off-center thrust, any type of stall will be bad.

I used to teach in a Piper Tomahawk, which had propellers that spun in the same direction, so it had a critical engine. There was a very noticeable difference between shutting down a left engine vs a right engine. I also flew a Beech Duchess, which had counter-rotating props. There wasn't a lot of difference between either engine, though the yaw was still very pronounced.

And lastly, I flew a KingAir (and its variants), which had some of the best single-engine performance of any prop-driven aircraft I've ever flown. Granted, it had all sorts of systems to help with an engine failure, but the PT-6s on that thing are fantastic engines. You could lose an engine coming off the runway and barely notice except for all the red lights that start flashing. :-)

One thing to add about yaw, that was one of the biggest reasons for feathering propellers (turning the blades so they pretty much point at the incoming airflow and minimize drag). A non-functioning propeller just about operates in reverse: it's like a great big parachute attached to the dead engine, making your yaw towards that side even worse. If you lose an engine, one of the first things you usually want to do is feather the dead prop. After you've identified the dead engine, of course. It really sucks to accidentally feather your good engine, though it does significantly reduce noise. :-)

Wow, old answer that apparently just got a vote. Anyway, when I looked at my comments, I was mistaken. I did teach in a Tramahawk, but I was confused. The Tomahawk was a horrible, low-wing single that was way too likely to spin when it stalled. The Aztec was the twin. I did my ME cert in that one and some teaching afterwards. That thing was fun when flying, but a beast if the left engine quit.


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How to Start Flying?
I am a teenager(mid teen) who wants to learn how to fly and recently discovered this website. I currently want to learn to fly just for my own pleasure. I don't exactly know what I want to do as a career but I am very interested in math and physics. I live in the Los Angeles area. How do I start my ambition? Plus, what would be the cost of learning how to fly?(airplane rent, fuel, instructor, etc.)

You have to run very hard first...:)

You may wish to consider flying gliders.

Sorry that this was shut down so fast without much encouragement, but there are many paths to take, many opinions, and no firm, concise, or correct answer for your specific situation.  @StephenS offered a great answer below that should at least point you in the right initial direction.  (you may try other on-line discussion forums to help you get advice)  Best wishes whatever you do!

First, take a “Discovery Flight” at a nearby flight school or flying club to determine if it’s something you enjoy as much as you think you will. Aim for morning or evening to avoid the bumpy air in the afternoons, especially in the summer.
Assuming that goes well, get your 3rd class medical certificate. This may take a few minutes or a few years, depending on your medical history. You don’t want to burn any further money on flight training until you have this in hand.
Next, figure out how you’re going to pay for flight training. Budget $10-15k just to get your Private Pilot certificate. There are a few scholarships out there, especially if you’re in a minority group, but it’s unlikely they’ll cover the entire cost or even most of it. Larger flight schools will have loan partners, but you’ll likely need to get your parents to co-sign, and they’re intended for people who plan on an airline career—and the full cost of the training for that is staggering. This is something you’ll want your parents’ help with figuring out.
Last is actually picking a place to train and signing up. Flight schools generally charge more per hour than flying clubs, but you’ll fly more often and thus need fewer hours to get your license, so the total cost is not as much more as it may appear at first glance.


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How are cabin pressure and oxygen level maintained inside the airplane?
How are cabin pressure and oxygen level maintained inside the airplane?

This is a pretty broad question with lots of information available here and elsewhere.  (I'm not going to research for you and post links...)  Have you already looked?  Is there something specific you don't understand?  (Short answer:  compressor stage bleed air, valves, and pressure regulators.  Oxygen ratio is same as ambient.)

If the pressure in the cabin of a pressurised transport aeroplane is maintained to under 10000 ft, the partial pressure of Oxygen is enough not to cause any significant
adverse physiological effects to humans. To achieve that, hot compressed air is taken from the engines and is passed in the cabin. There are also valves on the fuselage, which control the pressure and introduce fresh air into the cabin if needed.
In contrast most military fighter jets use a dedicated system to provide the pilot directly with oxygen. The two main types are OBOGS (onboard oxygen generating system) and pure oxygen supply through LOX bottles. They are usually supplied using ambient pressure, so above 40000 ft you need a oxygen supply under pressure, because the ambient pressure is not enough to sustain the diffusion of gases in the lung.
Some general aviation aircraft also possess a dedicated oxygen supply using LOX bottles for short flights above 10000 ft.


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Why doesn't the -30% SFC fully account for the dramatic reduction in fuel consumption compared to the 1960s?
In looking at the specific fuel consumption (SFC) of some aero engines I find that for example, the Rolls Royce Conway turbofan engine had typical cruise SFC of 0.874 lb/(h lbf) whilst the most modern equivalent, the Rolls Royce Pearl has a SFC of ~0.6.
This ~30% improvement in SFC over fifty years doesn't seem to account for the dramatic reduction in fuel consumption of new versus 1960’s aircraft that used the Conway. Is the rest accounted for by just the airframe improvements or am I missing something?

A rule of thumb is that two thirds of improvements are from better engines and the remaining third is from better aerodynamics.


text:
Are planes flying at night in Papua New Guinea?
I was watching the old TV show Destination Truth and the host Josh Gates said that no planes fly after dark in Papua New Guinea, is what he was saying fact?

The Civial Aviation Safety Authority (CASA) of Papua New Guinea allows flying at night, assuming the aircraft is equipped for night flying:

91.511 Night instruments and equipment
(a) Each powered aircraft with an airworthiness certificate operated by night shall be equipped in
accordance with 91.509 and have—

except as provided in paragraph (b), a means of indicating rate of turn and slip; and
position lights; and
an anti-collision light system; and
illumination for each required instrument or indicator or charts.

(b) An aircraft equipped with a third attitude instrument indicator that is usable through 360° of
pitch and roll does not need to be equipped with a means of indicating rate of turn.

(CASA PNG Part 91 General Operating and Flight Rules)
However, as @Pondlife pointed out in the comments, flying VFR at night is only allowed in the vicinity of the airport:

91.301 VFR meteorological minima
(a) Except as provided in rule 91.303 and paragraphs (b) and (c), a pilot-in-command must not
operate an aircraft under VFR— [...]

more than 5 nm from the aerodrome of departure at night.

Any flights further away will require flying under IFR at night.
Searching through the playback feature of flightradar24, I found a night flight from Moro to Port Moresby performed in a King Air just a few hours ago:

(flightradar24.com)
Sunset in Port Moresby was at 18:33 local time today and the aircraft departed at 19:16 and was estimated to land at 20:39.

The  episode was first aired in 2007 so maybe back then it was true, but today they have changed.

You might like to add that 91.301 says that night VFR is only allowed within 5nm of an airport. I know nothing about the TV show, but it's possible that they were exaggerating or generalizing that rule into "no night flying", especially if the show included any bush and/or mission flying.

@Pondlife Thanks, added.

To be fair, most countries don't allow night VFR - the U.S. is very much an exception in this regard.

My understanding is that IFR in PNG is basically VFR with traffic advisories over a terrible quality HF radio (because most of the airspace is actually uncontrolled?—not sure).


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Are airports still required to have a rotating beacon?
In the age of computers (GPS) do newly built airports still have the rotating beacon?

If an airport has at least one lighted runway, it will usually have a rotating beacon, regardless of when it was built. Equipment is expensive to maintain over time, so if they weren’t still useful, they’d all have been removed.
Even if pilots can easily navigate to an airport via GPS, we still must acquire it visually when coming from a direction that is not sufficiently aligned with a runway to recognize the runway or approach lights. This is quite common if we’re VFR entering the traffic pattern or if we’re IFR on a circling, visual or contact approach.

@757toga: There are a large number of airports that don't have beacons, or indeed, any sort of lighting at all.  Basically any back-country dirt strip, or low-use GA airports.  For the rest, some of us regard GPS as a useful backup, while doing primary navigation by looking out the window :-)

Thanks for your replies so far.

I thought a rotating beacon meant there was an operating control tower at the field.

@MichaelHall Every airport I’ve seen with runway lights has a rotating beacon, including non-towered airports.

Sure enough, I see a whole bunch on the chart.  Dang, not sure where I got that idea from now...

How about airports in other countries?


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Is an increase in induced drag always caused by an increase in the angle of attack?
In this video at 2:57 they increase the AoA to maintain lift, which normally also increases the drag. Now, imagine that we don't increase the AoA: the drag will not increase but we will not have enough lift to maintain level flight. If we want to keep the same lift and fly in level flight we must increase the AoA and only then will the drag increase.
So induced drag is just caused by a larger AoA, so basically this is a type of pressure drag.
Do you agree that if we don't increase the AoA as described in the video then the drag will not increase as well?

This question could benefit from being cleaned up a little.  Can you pin it down to just one single question?  Try not to invalidate the existing answer though--  Maybe on last paragraph delete all but the first sentence, since it is clear enough that two wings generating different amounts of lift will normally generate different amounts of drag.

Welcome to aviation.SE! I edited your question to (hopefully) make it clearer, and to help you get a better answer. If I made a mistake or changed it too much, you can edit it again yourself or roll back my changes. The [tour] might also be helpful if you're new to the site.

At 2:58, the video says "To replace the lift lost by the increased upwash and downwash, the wing must be flown at a higher-angle-of-attack than would otherwise be necessary."
This statement is true no matter what angle-of-attack the wing is "feeling" and no matter how much lift it is making, as long as the amount of lift it is making is not zero.
For example, let's say at a given instant in time, with a given angle-of-attack and airspeed, when the instantaneous flight path is horizontal, the wing would be making Lift equal to Weight if it weren't for the upwash and downwash effect, but because of the upwash and downwash effect, the wing is only making a force equal to 90% of Weight.
Forces are not in balance, and the flight path is curving downward.  In the next instant in time the flight time will no longer be horizontal.  Yet despite the fact that we aren't actually replacing the "missing" lift, it is still true that that "To replace the lift lost by the increased upwash and downwash, the wing must be flown at a higher-angle-of-attack than would otherwise be necessary."  If we were happy to settle for a lift force equal to 90% of weight at that instant in time, and the upwash and downwash effect were not there, we could fly at an even lower angle-of-attack.
More practically, we normally don't want to allow the flight path to curve (accelerate) downward, so we do keep lift close to weight, or very close to it, except during the brief transition (pushover) from a climb to level flight or from level flight to a descent.  The reduction of lift that happens in a steady-state descent (or climb), compared to in level flight, is very small for moderate climb or descent angles.  For more, see a What produces thrust along the line of flight in a glider? and Does lift equal weight in a climb? .
Only if the wing is mounted inside a wind tunnel, where the connection between Lift and Weight is completely severed, does it really make any sense to ask "What happens if we decide not to replace the missing lift?".  In that case, indeed, the upwash/downwash effect does not force the wing to be flown at a higher angle-of-attack than would otherwise be necessary.

Yes agree with you, when plane fly than it doesnt make sense ask "what if we dont replace missing lift"because plane will descend in that case ..But I ask this question for better understand what induced drag really is.Because induced drag will not increase/dont exist if we dont increase AoA.Look at my new topic so here you can post your opinion,link: https://aviation.stackexchange.com/questions/77573/will-3d-wing-has-smaller-drag-than-2d-at-same-aoa-and-airflow-speed

It is hard to understand the video in my opinion and also it is hard to understand the induced Drag mechanism thoroughly. I would like to recommend chapter 5 of Adams's "Fundamentals of Aerodynamics" book or other Aerodynamics book to understand in detail.
I will try to explain it as well as I can by myself.
Lift (C_L) is the result of a pressure Difference (cp_down - cp_up) between the upper and lower surface. Because of this pressure difference, the air is trying to go from the high pressure lower surface to the low-pressure upper surface. When reaching the tip of the wing it has to curl around, which creates the Vortex. Therefore, Higher Lift results from higher pressure difference and results in higher vortex strength at the tip. The pressure difference/Lift coefficient is usually higher for higher AoA, and lower for AoA.
When you fly slower, you need a higher CL, which results in a multiple Times higher CDi. Which means high induced Drag at slow flight.
Unfortunately, there are more ways to explain the induced Drag.
Because of the vortex at the tip, a downwash (velocity in the z-direction) is generated. This downwash is changing the local effective aerodynamic AoA, which is different from the local geometric AoA(you can see the difference in the video 7:05). Because the, resulting aerodynamic force is always perpendicular to the air velocity(which has now an additional z component), the Lift vector has then an additional x component. This additional x-Component is the induced Drag(video 3:03).
Another way to understand the induced Drag would be:
The 2D Flow is being changed by the vortex in a 3D flow, which is changing the pressure distribution of the wing in such a way that a pressure Difference in front and behind the wing exists. So it is a "pressure Drag".
Is an increase in induced drag always caused by an increase in the angle of attack?
Drag = Drag Coefficient * dynamic Pressure * Wing surface
No! The induced Drag can rise with higher velocity or higher density, too(if AoA is constant). But usually the needed Lift coefficient is sinking with higher dynamic pressure and therefore the Drag Coefficient too.

I like this description from Anderson -Fundametals of aerodynamics":The three-dimensional flow induced by the wing-tip vortices  simply alters the pressure distribution on the finite wing
in such a fashion that a net pressure imbalance exists in the direction of V∞
(i.e., drag is created). In this sense, induced drag is a type of “pressure drag.”


text:
Part 65.61.d.2 IFR Requirements
I have a question regarding this requirement as I work on my IFR SEL rating:

Forty hours of actual or simulated instrument time in the areas of
  operation listed in paragraph (c) of this section, of which 15 hours
  must have been received from an authorized instructor who holds an
  instrument-airplane rating, and the instrument time includes [...]

The question is, if 15 hours of the 40 hours is required to be with an authorized instrument rated instructor, then how do you gain the other 25 hours of the required 40 hours?  Wouldn’t all 40 hours have to be with an instrument rated certified instructor?

Welcome to Av.SE

Opinions differ on whether it is a good idea to fly without an instructor, but I think that after you have mastered the instrument scan, power settings and sppeds, reading the approach plates, and talking with ATC the instructor is basically acting as a safety pilot. Flying with another instrument student would allow both of you to learn at the same time. It is helpful to me to watch other pilots fly IFR and it may be useful to you too. If I was to do my IFR over, I’d set up a GoPro in the cabin and review my flights.

If you're under the hood in VMC + VFR, you could have an appropriately rated pilot (i.e. PPL) in the other seat as the safety pilot. If the other pilot is instrument rated (but not a CFII) + is the PIC for the flight, you could even be the sole manipulator of the controls in IMC + IFR, although that gets into the questions about logging PIC vs acting as PIC, which is its own set of issues. The "safety pilot" option is probably more typical.

Thank you Ralph! That does make sense, and that information helps!


text:
What is the definition of "geometric pitch angle" for a propeller?
My question is regarding the geometric pitch angle of a propeller. Is it the angle between the rotational/propeller plane and the zero lift line of the particular airfoil at a propeller section? Or is it between the propeller plane and the chord line of the airfoil?

From a variety of online sources (e.g. 1 and 2), the geometric pitch angle is simply the incidence angle between blade reference chord and the plane of rotation.
This makes sense from the definition of blade pitch, which is the distance that a point on the prop would move forward in one rotation, if it's gripping in a solid medium. The conversion from pitch to pitch angle, therefore, has nothing to do with aerodynamics (i.e. lift) and is geometric only.


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What does Region @TOP mean on the aviationweather.gov website?
When looking at raw METAR data on the aviation weather center, "@TOP" appears in the text box where you can type identifiers. What does TOP mean?

In this case, @TOP is the output from the top U.S. airports. You can enter two letter state abbreviations (i.e. @NY, @MO) or country abbreviations (i.e. #GB, #DE). (https://www.aviationweather.gov/help/tutorial)

@TOP is a special code on the aviationweather.gov website. It shows data for the top U.S. airports around the country.  This isn't the only special code that can be placed in the box.  You can also use the two character identifier for the state to see all data for that state.  See the fourth row below.


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What is the helicopter in the background of this image of the V-22 Osprey?
I found a photo of a V-22 Osprey alongside an unknown helicopter on the left corner:

This is Blackout, the coolest Transfomers' Decepticon =)

It's an MH-53 Pave Low.  In the Air Force Special Operations Command (AFSOC), the CV-22 replaced the MH-53, although there were several years when both were in the inventory.
From the look of the beach, this photo was taken near Hurlburt Field, Florida, which is where they were both based as the first CV-22's came on-line.  Hurlburt Field is also the headquarters of AFSOC.

That looks like a Sikorsky MH-53 to me.

Airman Magazine, MSgt Dave Nolan / Public domain

One of the countless H-53 variants and sub-variants, most likely a MH-53J/M.


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What were those two military helicopters that I recorded?
I have recently recorded two military helicopters. Here is a screenshot of it:

Geo location will help because military models have relatively predictable area of operation.

It is hard to tell from such a low resolution picture. Due to the shape of the narrow body, they look like USMC Bell AH-1 SuperCobra. It is probably the AH-1Z Viper version based on the four blade rotor.


text:
Can I count solo complex time as complex training time for a commercial certificate?
Does a combination of both dual and solo hours in a complex aircraft (PA28R-180) which total 10 or more flight hours meet the commercial pilot training requirements of FAR 61.129(a)(3)(ii) or must all the hours be dual instruction?

As you already noted, 61.129(a)(3)(ii) says "training" (emphasis mine):

(ii) 10 hours of training in a complex airplane, a turbine-powered
airplane, or a technically advanced airplane (TAA) that meets the
requirements of paragraph (j) of this section, or any combination
thereof. [...]

Here's the definition of training in part 61, per 14 CFR 61.1:

Training time means training received—
(i) In flight from an authorized instructor;
(ii) On the ground from an authorized instructor; or
(iii) In a flight simulator or flight training device from an
authorized instructor.

According to that definition training always requires an instructor so there's no way you could log solo time as training time. All 10 hours must be dual instruction.

It has to be dual instruction per 14 Title CFR  PART 61.129:

§61.129 Aeronautical experience.
(a) For an airplane single-engine rating. Except as provided in paragraph (i) of this section, a person who applies for a commercial pilot certificate with an airplane category and single-engine class rating must log at least 250 hours of flight time as a pilot that consists of at least:
(3) 20 hours of training on the areas of operation listed in §61.127(b)(1) of this part that includes at least—
(ii) 10 hours of training in a complex airplane, a turbine-powered airplane, or a technically advanced airplane (TAA) that meets the requirements of paragraph (j) of this section, or any combination thereof. The airplane must be appropriate to land or sea for the rating sought;

So the 10 hours of a combined complex, turbine, and TAA time must be in an airplane and as part of training.